Have you ever heard some medicine or infectious disease keener throw out a “brown recluse bite” on their differential diagnosis for what looks like a simple insect bite? Have you seen a necrotic wound where a staff says it was likely due to a “spider bite”? Have you ever heard of Brown Recluse spiders? Do you live in Canada?
Below is an article about why it is exceedingly unlikely that your insect bite was a brown recluse, if you live in Canada. This is the first in a series of collaborative blog posts related to insects and infectious disease with my good friend Scott Willis.

To kick things off lets look at Table 1 from Swanson and Vetter, NEJM 2005 of conditions potentially misdiagnosed as bites from loxosceles spiders. This table always makes me smile.
David L. Swanson, M.D., and Richard S. Vetter, M.S. Bites of Brown Recluse Spiders and Suspected Necrotic Arachnidism. N Engl J Med 2005; 352:700-707. February 17, 2005.
Brown recluse spiders (Loxosceles reclusa) are small, brown spiders that have a potentially serious venomous bite that can cause skin necrosis (or at least that is what everyone is worried about). While this can be very startling, residents of Canada do not have to fear about being bitten by a brown recluse. The brown recluse spider can only be found in south Central/Midwest United States.

Wait, hold the press! What about a brown recluse which hitches a ride on a load of fruit shipped from the United States?

This question always gets raised and becomes a question of probabilities. I ask you to consider which is more likely…that a lone loxosceles spider hitch-hiked thousands of kilometers waiting stealthily to bite one unlucky soul…or that someone might not be using a suitably wide differential diagnosis (my money is on the later). Remember we should cast a wide net when formulating our diagnosis rather than settling on a diagnosis of convenience with little actual concrete evidence.

Despite this, many doctors in Canada still diagnose brown recluse bites. The belief that brown recluse spiders are in Canada is an urban legend; only 3 have been ever verified in Canada. Even having the spider accidentally transported from its natural habitat to Canada is extremely rare. The spiders themselves rarely bite, and are very shy in that they are only active at night. They prefer rocks and crevices, are not found in the open, and avoid human contact. They will only bite when trapped between you and a surface (putting on shoes with the spider inside for example).

Many times a patient will have their bite misdiagnosed as a brown recluse bite without anyone ever seeing a spider. Some states that do not have natural recluse populations have guidelines that state that if the spider is not seen in the act of biting then it should be treated accordingly and not as a necrotizing spider bite. Thus, it is important to have your patient bring in the offending spider if they are bitten, or at the very least they must report seeing the spider.

The diagnosis really comes down to whether the spider that bit the patient was a brown recluse or not. Brown recluse spiders are the most misidentified spider by amateurs and experts alike.

So you were bit by a spider, but was it a brown recluse?

Most people would consider the “violin head pattern” on the “middle” of the spider to be the most unique feature. Unfortunately most people identify the spider wrongly on this trait, as many other spiders have patterns that might resemble a violin head. The pattern itself on the recluse is not always vibrant or present, and can only be found on adult spiders. While brown recluse spiders are known for this pattern I would not recommend using it to identify the spider. If it is present, it can reinforce your identification, but it should not define it.

“Violin Pattern”
The number of eyes and their arrangement on the spider is crucial to a proper identification. Most spiders have eight eyes. Brown recluse spiders only have six. This is rare, as there are few spiders that only have six eyes, none of which resemble the brown recluse. The arrangement of the eyes is also important.

Most other spiders have their eight eyes arranged in two rows of four like this:

One pair is in the middle, the other two are on each side. If you have a spider with six eyes in this arrangement, you have a brown recluse.

There are a few other distinguishing characteristics that would help identify a brown recluse. Brown recluse spiders’ legs have a uniform light colour to them and there are no stripes or bands on the legs. Also, the legs have no spines, only fine hairs. The size of the spider is only about 6-20mm in size. If the spider meets all of these characteristics, it is a brown recluse spider. In a pinch though, the eyes are the key.

While the above information allows you to identify a brown recluse, we must stress that the bite isn’t a brown recluse bite. While the bites can be traumatizing, a diagnosis should only occur in regions and countries where Brown Recluses populate and live.

Consider other bugs as source of bite (unless presented otherwise); fleas, ticks, mites, bedbugs and assassin bugs. These bugs actually seek out humans, whereas spiders do not. If you can capture the spider, send it to an arachnologist for identification. Even if you get bit, a necrosis occurrence is very rare, occurring about 37% of the time. Most bites heal without any problems.

All of this, including the shyness and unwillingness of the spider to bite except in extreme circumstances makes a bite from a brown recluse in Canada – EXTREMELY UNLIKELY.